As an example of an optical information recording medium, there can be mentioned a phase-change information recording medium with respect to which information can be recorded, erased, and rewritten by an optical means using a laser beam. Blu-ray Disc media, for example, are now commercially available as a phase-change information recording medium. Single-side, dual-layer BD-RE (Blu-ray Disc Rewritable) media (with a recording capacity of 50 GB and a transfer rate of 36 Mbps (1× speed)) are commercially available as a high-capacity medium that can be used for high-definition digital broadcasting.
Such single-side, dual-layer BD-RE media have two information layers, in which a fully reflective information layer L0 located farther from the laser beam incident side and a semi-transmissive information layer L1 located on the laser beam incident side are provided with an optical separation layer interposed therebetween. As an example of the configuration of the semi-transmissive information layer L1, there can be mentioned a configuration in which a transmittance adjusting film, a reflective film, a first dielectric film, a first interface film, a recording film, a second interface film and a second dielectric film are formed in this order on a surface of the optical separation layer.
For the recording film, a phase change material capable of causing a reversible change between the crystalline phase and the amorphous phase is used. Examples thereof include a material having a composition on a line extending between Ge50Te50 and Sb40Te60 (see Patent Literature 1), a material having a composition on a line extending between Ge50Te50 and Bi40Te60 (see Patent Literature 2) obtained by substituting Sb with Bi in the above-mentioned material, and a material that contains Sb as its main component (about 70 atom %) and has a composition in the vicinity of an eutectic point of SbTe as a base (see Patent Literature 3). Generally, information is recorded as follows: the recording film is heated to a temperature higher than its melting point by irradiation with a laser beam at high power (recording power), so that the irradiated region is melted, followed by rapid cooling, thereby forming an amorphous phase. In contrast, information is erased as follows: the recording film is heated to a temperature higher than its crystallization temperature but lower than its melting point by irradiation with a laser beam at lower power (erasing power) than in recording, so that the temperature of the recording film is increased, followed by gradual cooling, thereby forming a crystalline phase. A reflectance difference occurs between the crystalline region and amorphous region thus formed, which enables information to be reproduced. The larger the reflectance difference and the reflectance ratio between the crystalline region and the amorphous region, the higher the quality of reproduced signals to be obtained.
The first dielectric film and the second dielectric film have functions of enhancing the optical absorption efficiency of the recording film by adjusting the optical distance (=refractive index×physical distance), increasing the difference between the reflectance in the crystalline phase and the reflectance in the amorphous phase, and increasing the signal amplitude. Further, they also have a function of protecting the recording film from moisture, etc. Examples of the material for these dielectric films include a mixture of 80 mol % ZnS and 20 mol % SiO2 (hereinafter, expressed as (ZnS)80 (SiO2)20(for example, see Patent Literature 4 and Patent Literature 5). This material is an amorphous material and has low thermal conductivity, high refractive index, and high transparency as its properties. Further, it has a high deposition rate in film formation and has excellent mechanical properties and moisture resistance as well. Because of such excellent properties, (ZnS)80 (SiO2)20 has been put to practical use as a material that is very suitable for forming a dielectric film.
The transmittance adjusting film has a function of adjusting the transmittance. Use of a material with a high refractive index n (preferably n≧2.5) for this film enhances the transmittance of the information layer. Further, it allows the transmittance of the information layer when the recording film is in the crystalline phase and the transmittance of the information layer when the recording film is in the amorphous phase to approximate each other.
The first interface film and the second interface film are provided to prevent the elements that constitute the first dielectric film and the second dielectric film from diffusing into the recording film when rewrite-recording is performed repeatedly, and to prevent the rewriting properties of the recording film from being changed. As a material for the interface films, there has been disclosed a material containing ZrO2 and Cr2O3, for example (see Patent Literature 6). This material is excellent because it has high transparency with respect to a laser in the blue-violet wavelength region (near 405 nm) and also has high heat resistance due to its high melting point.
Optically, the reflective film has a function of increasing the amount of light to be absorbed by the recording film. Thermally, the reflective film has a function of rapidly diffusing heat generated in the recording film and rapidly cooling the recording film so as to make it easy for the recording film to be amorphous. Furthermore, the reflective film also has a function of protecting the recording film, the interface films, and the dielectric films from the use environment. Therefore, Ag alloys with high thermal conductivity have been preferably used as a material for the reflective film.
In the future, it will be necessary to increase the capacity per disc in response to a further increase in volume of recording contents, widespread use for personal computers, resource savings in view of environment, and space savings.
The capacity per disc can be increased, for example, by further increasing the number of information layers to be stacked (multi-layering), or further increasing the recording capacity per information layer (recording density increase).
For achieving multi-layering of information layers, it is necessary to increase the transmittance of semi-transmissive information layers located nearer to the laser beam incident side so that information (with high S/N ratio) can be correctly read out also from the information layer located farther therefrom. In order to enhance the transmittance of the information layers, it is necessary to design the recording film and the reflective film, which have high absorptivity of laser beams, to be thin in the aforementioned configuration. However, if the thickness of the recording film and the reflective film is reduced while the reflectance of each information layer is maintained, the reflectance ratio decreases, resulting in deterioration of the signal quality or degradation of the rewriting performance. Accordingly, there is a limit in reducing the thickness of the recording film and reflective film. Therefore, in a multilayered information recording medium, as the number of information layers to be stacked increases, the reflectance of the information layer is set to be lower and the power of the reproducing beam is increased more at the same time so that signals with high S/N ratio can be recorded/reproduced.
For increasing the recording capacity per information layer (in-plane), it also is conceivable to increase the recording (mark/space) density, leading to an increase in the recording capacity per information layer, for example, from 25 GB to 30 GB or 33.4 GB. The increase in the recording density causes the interval of a recording mark (amorphous region) and a space (crystalline region) to be reduced. Thus, the material composition of each layer and the configuration of the information recording medium are required to be made suitable for high density recording.